you can drill anything you want out of your bass, as long as youve got your pickguard over it to cover it up afterwards. i had to cut a new slot from scratch for a new pickup i put in. its not hard to do if you know what your doing, plus youve got most of the work done already. my advise is to just drill out what you need-a little at a time- and then put thick foam underneath to make sure the pups will be steady if they are on uneven ground. the hardest part for you will be making sure you drill out your pickguard so it matches up exactly with the dimensions of your pups. whatever you do, just be careful not you hurt yourself or your bass in anyway you dont want to.

Ok, so first you had a "P-Bass" and now you have "J-Bass". I'm confused. No matter what, you can put any pickup in any bass, but it is all custom work. Many of us here have done custom work on our basses and there is plenty of support out there for when you're ready to do this.

Before you do this, you need to figure out what kind of bass you actually have, and then choose what pickups you want to fit.